Scientists Can Detect Your Sleeptime Memories Even If You Can't

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A few words might be getting through in your sleep.

For the very ambitious who want to capitalize on the one-third of their lives spent asleep, learning while snoozing is the ultimate dream. Just imagine if you could learn a new language by streaming podcasts into your sleeping ears. And then another language, and then another.

That unfortunately is still a dream. But in a new study, published Aug. 24 in the journal Neuroscience of Consciousness, researchers found that faint traces of memory formed during sleep even if the individuals themselves remained largely unaware of those memories.

In the study, neuroscientists Thomas Andrillon and Sid Kouider of Paris’ Ecole Normale Superieure asked 22 participants to listen to an audio stream of words as they drifted into sleep. The words kept playing in their ears during the nap.

Upon awakening, participants did not remember the words they had heard while asleep and couldn’t tell them apart from new words in a memory test.

However, using electroencephalography, which picks up on the brain’s electrical activity, the researchers found an implicit memory trace for words presented during sleep: Hearing those words after the nap triggered a EEG result different from that of completely new words.

The scientists could correctly determine whether the participant’s brain had heard a certain word during sleep 55 percent of the time ― again even though the participant had no memory of it.

Of course, 55 percent accuracy is hardly a reliable crystal ball for mind reading but it’s still above the level of chance, Andrillon said.

Moreover, EEG data are only an approximation of brain activity. It’s akin to trying to listen in on people’s conversations from outside a bar. You can’t hear much but can pick up on the occasional collective cheer.

The finding that words heard in sleep leave a trace behind was corroborated by measuring participants’ confidence level in memory tests. When participants were asked if a certain word was presented to them during sleep, they expressed a bit more confidence in their answer when it was correctly yes — as if they had a hunch about that word.

Scientists know that during sleep, the brain reviews and stores information learned throughout the day. This nighttime housekeeping, it’s been thought, keeps the brain too busy to form any new memories of what happens during sleep itself.

But the new findings suggest that some minimal memory formation is possible. Still, that doesn’t mean you can learn new information during sleep in any useful way. If anything, the memory traces were so weak that one could conclude the opposite: that it’s really hard, if not impossible, to learn anything useful during sleep.

In other words, Andrillon said, the potential benefits of nighttime learning are not worth losing quality sleep over.